Typical petroleum drilling operations employ a number of techniques to gather information about the borehole and the formation through which it is drilled. Such techniques are commonly referred to in the art as measurement while drilling (MWD) and logging while drilling (LWD). As used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) and often includes information about the size, shape, and direction of the borehole. LWD typically refers to measurement taken for the purpose of analysis of the formation and surrounding borehole conditions and often includes various formation properties, such as acoustic velocity, density, and resistivity. It will be understood that the present invention is relevant to both MWD and LWD operations. As such they will be referred to commonly herein as “MWD/LWD.”
Transmission of data from a downhole tool to the surface is a difficulty common to MWD/LWD operations. Mud pulse telemetry is one technique that is commonly utilized for such data transmissions. During a typical drilling operation, drilling fluid (commonly referred to as “mud” in the art) is pumped downward through the drill pipe, MWD/LWD tools, and the bottom hole assembly (BHA) where it emerges at or near the drill bit at the bottom of the borehole. The mud serves several purposes, including cooling and lubricating the drill bit, clearing cuttings away from the drill bit and transporting them to the surface, and stabilizing and sealing the formation(s) through which the borehole traverses. In a typical mud pulse telemetry operation, a transmission device, such as an electromechanical pulser or a mud siren located near the drill bit generates a series of pressure pulses (in which the data is encoded) that is transmitted through the mud column to the surface. At the surface, one or more transducers convert the pressure pulses to electrical signals, which are then transmitted to a signal processor. The signal processor then decodes the signals to provide the transmitted data to the drilling operator.
One significant difficulty with decoding a mud pulse signal is the poor signal to noise ratio that results from both low signal amplitude and high noise content. For example, the amplitude of a transmitted pressure pulse tends to attenuate as it travels up the drill pipe. Such attenuation is dependent on many factors including the depth of the borehole, the type of drilling mud, the hydrostatic pressure, the number of joints in the drill string, and the width of the pressure pulse. Moreover, there are a number of potential sources of noise generated during drilling operations including turning of the drill bit and/or drill pipe in the borehole, sliding and/or impact of the drill pipe against the borehole wall, and the mud pump that is used to pump the mud downhole.
Due in part to the poor signal to noise ratio, data transmission rates are slow (e.g., on the order of about 1 bit per second). Increasing the transmission rate tends to decrease the signal to noise ratio due to decreased signal amplitude. The low signal to noise ratio also tends to increase the frequency of transmission errors which can erode the reliability of the communication channel and disrupt the synchronization between the downhole encoder and the surface decoder. As is known to those of ordinary skill in the art, these problems can be severe in ordinary drilling operations, and in particular in deep wells.
U.S. Pat. No. 4,908,804 to Rorden discloses a combinatorial coding technique for mud pulse telemetry. While the methods disclosed in the Rorden patent have been commercially utilized, there remains room for further improvement. For example, there remains a need for coding and decoding methodologies that improve both the efficiency and reliability of mud pulse telemetry communications.